1. Field of the Invention
The present invention relates to a surface mount antenna and a communication apparatus using the same, more particularly to a surface mount antenna used in a mobile telephone and a communication apparatus using the same.
2. Description of the Related Art
Conventionally, a whip antenna, capable of obtaining a wide pass band for covering both transmitting frequency and receiving frequency bands, has principally been used as the main antenna of a mobile telephone. However, since a whip antenna protrudes from the case of the mobile telephone, it is bulky and liable to break, and progress in development of small-scale and lightweight mobile telephones has brought a need for a small-scale antenna covering a wide pass band and which is not bulky.
FIG. 9 shows a conventional antenna aimed at obtaining a wide pass band. In FIG. 9, an antenna 1 comprises several electrodes provided on faces of a rectangular box-shaped base 2, which is an insulator comprising a dielectric such as ceramic or resin. Firstly, a ground electrode 3 is provided almost entirely over a first main face of the base 2. Furthermore, a first radiation electrode 4 and a second radiation electrode 5 are provided in parallel, with a gap g1 in between them, on a second main face of the base 2. Furthermore, one end of the first radiation electrode 4 forms an open terminal, and the other end crosses over (extends) to the first main face via one of the end faces of the base 2 and connects to the ground electrode 3. Furthermore, one end of the second radiation electrode 5 forms an open terminal and the other end crosses over (extends) to the first main face, via the same end face of the base 2 as in the case of the first radiation electrode 4 and connects to the ground electrode 3. Then, a feed electrode 6 is provided in another end face, opposite to the end face of the base 2 which the end faces of both the first radiation electrode 4 and the second radiation electrode 5 cross over (extend) to, and one part of the feed electrode 6 crosses over (extends) to the first main face of the base 2.
In the antenna 1 of such a constitution, when a signal is transmitted to the feed electrode 6, capacitance between one end of the first radiation electrode 4 and the second radiation electrode 5 and the feed electrode 6 transmits the signal to the first radiation electrode 4 and the second radiation electrode 5. Then, since one end of the first radiation electrode 4 and the second radiation electrode 5 becomes an open terminal and the other end becomes a connection terminal, the electrodes 4 and 5 are resonant at a frequency where the length from the one end to the other end is a quarter of the effective wavelength. Now, the pass band of the antenna 1 can be made wide by differing the resonant frequencies of the first radiation electrode 4 and the second radiation electrode 5 so that their pass bands overlap slightly.
However, in the antenna 1 shown in FIG. 9, the gap g1 is narrow in order to ensure that vectors of the resonant currents flowing through the first radiation electrode 4 and the second radiation electrode 5 are parallel, but when the resonant frequencies of the first radiation electrode 4 and the second radiation electrode 5 differ considerably, only one of the radiation electrodes is resonant and the other radiation electrode is not resonant, making it difficult to achieve a stable double resonance. Furthermore, when the antenna 1 is made small-scale by reducing the gap g1, the two radiation electrodes are moved closer to each other, whereby current flows through the two radiation electrodes in reverse phase, causing further deterioration of antenna characteristics.